1. Field of the Invention
The present invention relates to a voltage controlled oscillator and a method of generating an oscillating signal. More particularly, the present invention relates to a temperature independent voltage controlled oscillator and a method of generating an oscillating signal that may generate an oscillating signal having a stable frequency independent of temperature variation.
2. Description of the Related Art
A voltage controlled oscillator (VCO) generates an oscillating signal having a frequency corresponding to an input voltage, and is used in a phase locked loop (PLL) as to generate oscillating signals.
FIG. 1 is a circuit diagram showing a conventional voltage controlled oscillator.
As shown in FIG. 1, a current mirror 100 of the conventional VCO generates a current 12 corresponding to a control voltage provided from an external source, and a ring oscillator 110 generates an oscillating signal having a frequency corresponding to the current 12. A buffer 120 stabilizes the oscillating signal and outputs a stabilized oscillating signal Fout.
The frequency of the oscillating signal Fout is proportional to the current I2.
Typically, the current I2 decreases as temperature increases. Accordingly, the frequency of the oscillating signal Fout decreases as temperature increases. As a result, a gain [Hz/V] of the VCO decreases as the temperature increases. The gain is referred to as a frequency to control voltage ratio.
Threshold voltages of the transistors M2 and M3 of the current mirror 100 decrease as temperature increases. Thus, the current I2 generated by the current mirror 100 decreases as the temperature increases.
FIG. 2 is a graph showing frequency variations of the voltage controlled oscillator of FIG. 1 according to temperature variation when the control voltage Vctrl is changed from 0 to 1.8 volts. Curve ‘A’ of FIG. 2 shows frequency variation when the temperature is −55° C.; curve ‘B’ of FIG. 2 shows frequency variation when the temperature is 55° C.; and curve ‘C’ of FIG. 2 shows frequency variation when the temperature is 125° C.
Referring to FIG. 2, the frequency of the oscillating signal Fout decreases as temperature increases, and thus the gain (slope of the curve in FIG. 2) of the VCO decreases as the temperature increases.
Therefore, the frequency of the oscillating signal generated from the conventional VCO may vary depending upon temperature variation.
As a result, the conventional VCO may not generate a signal at the desired frequency when the conventional VCO operates in semiconductor chip of a high speed digital system that generates a lot of heat.